This invention relates to a method and a device for extracting the phase of a signal, possibly comprising noise, or a phase reference of a modulated signal, possibly comprising noise.
It is used particularly in applications in the domain of RFID (Radio Frequency IDentification) systems. An RFID system is usually formed from a first device containing stored data, usually called label, capable of transmitting these data in the form of radio waves at a frequency within a range usually between a few kHz and a few MHz. A second read device is then provided for acquisition of said stored data. To achieve this acquisition, the second device is capable of emitting a signal S formed from a carrier. In response to this signal S, the first device then emits another signal S′ containing said data. This other signal S′ is usually formed by a sub-carrier with a frequency proportional to the frequency of said carrier, according to a predetermined protocol. The other signal S′ may be modulated, for example using a phase modulation.
The read device should preferably be capable of detecting a phase reference of this other signal S′, so that the read device can demodulate the other signal S′. To achieve this, a number of cycles may be specifically inserted at the beginning of the other signal S′ so that the read device will have the time to detect this phase reference. One known method called the “correlation method” is currently used to do this detection type. This method consists of multiplying one or several cycles of said other signal S′, for which the phase reference is searched by one or several cycles of a reference signal S0 with a frequency identical to the frequency of said sub-carrier.
The analogue or digital results obtained from these multiplications are integrated over one or several periods. These operations are repeated regularly in time and are a maximum at the instant at which the other signal S′ and the reference signal S0 are in phase. The moment at which this maximum occurs corresponds to an instant used as the phase reference for the other signal S′.
This method has the disadvantage that it lacks precision, particularly when the other signal S′ comprises noise. It also requires the use of a large number of resources in the read device.